Molecule-based photonic materials represent a promising direction in the quest to develop novel electro-optic (EO) modulators promising greatly increased rates of information transmission by enhancing optical network speed, capacity, and bandwidth for data networking and telecommunications. Non-centrosymmetry is one of the basic requirements of these materials. Currently, three major methodologies are being used to achieve molecular orientation: electric-field (EF) poling, Langmuir-Blodgett (LB) film transfer, and layer-by-layer self-assembly (SA). In the first one, nonlinear optical (NLO)-active chromophores are either doped in or covalently bonded to a polymer to fabricate films. A high external electric field is then applied while the films are heated to around the glass transition temperature (Tg) to cause the chromophore dipoles to align in the direction of the electric field. It is a straightforward procedure to fabricate thick-poled films. However, the drawbacks are: 1) the orientation achieved by EF-poling is not indefinitely stable after removal of the EF; 2) due to strong dipole-dipole interactions among the chromophore molecules, the doping concentration cannot be brought to a high level; 3) micro-domains formed during EF-poling can increase the optical loss in a waveguide device.
For the LB film approach, only limited chromophores with long alkyl groups can be used. Since weak van der Waals interactions are the main structural driving force, the orientation becomes progressively worse as the film becomes thicker (e.g., after 100 layers). Other drawbacks include low NLO response and poor mechanical strength. For covalent self-assembly, the NLO response is strong, orientation is stable, and film quality is good. However, the main disadvantage is the time-consuming nature of the fabrication procedure (hundreds of hours might be used to achieve a micrometer thickness film). Additional synthetic complexity arises from use of moisture-sensitive reagents.
Although H-bonds are widely used in crystal engineering, the prior art is not directed to thin film deposition using H-bonding constituents. Since thin acentric films are needed for EO modulators, efficient new depositions methods would be of great utility. Dipolar orientations driven by H-bonds have been reported in drop-cast films. However, the H-bonding modules come from two different compounds (FIG. 1, structure A), and the films obtained are composites, and not derived from the vapor phase. A technique known as “oblique incidence organic molecular beam deposition” was also reported to produce oriented films with single H-bonds used to align chromophore molecules (FIG. 1, Structure B). However, the molecular dipoles are parallel to the substrate. Only in-plane directional ordering is achieved (FIG. 1, structure B). As is well known, in a waveguiding EO modulator device, the molecular dipoles must be oriented perpendicular to the substrate plane so that maximum EO coefficient, r33, can be achieved.
Vapor deposition techniques have previously been used in the art to fabricate ordered NLO organic films, such as stilbazolium salts, polydiacetylenes, etc; however, the driving forces do not involve H-bond formation. In stilbazolium salt films, the chromophore is generated in situ, and in ordered polydiacetylene films, van der Waals interactions play important roles. Reaction considerations limit the former, while unstable structural orientations plague the latter. As a result, the art continues its search for a facile assembly of robust films of NLO-active chromophores.